1. Field of the Invention
The present invention relates to an asynchronous transfer mode (ATM) exchange that performs communication by utilizing an available bit rate (ABR) service, and more particularly the present invention relates to an ATM exchange of a configuration in which a virtual destination and virtual source are constructed within a control loop.
2. Description of Related Art
An ABR service is a communication service in which the communication rate fluctuates according to the traffic on a network. With an ABR service, a minimum cell rate (MCR) and a peak cell rate (PCR) are set in the connection of a sending terminal and a network. A sending terminal is able to send at a communication rate equal to or greater than the MCR. If there is room in the network resources, the sending terminal can increase the communication rate, as long as the PCR is not exceeded.
An ABR service uses traffic control cells called resource management cells (RM cells) in order to determine how congested the network is. The sending terminal sends one RM cell every time a specific number of user cells are sent to a receiving terminal. This RM cell is called a forward RM cell (FRM cell). The receiving terminal obtains information about network congestion from the reception status of the user cells. Every time an FRM cell is received, the receiving terminal appends congestion information to this FRM cell and sends it back to the sending terminal. The returned RM cell is called a backward RM cell (BRM cell). The rate that is acceptable to an ATM exchange or other such device is sometimes clearly written to a BRM cell when it passes through such a device. The sending terminal controls the sending rate by using the information it reads from the received BRM cells.
Because the traffic control loop between the sending terminal and receiving terminal is extremely long, there is sometimes a problem in that it takes longer to communicate an RM cell. An ATM switch is sometimes used as an ATM exchange in order to solve this problem. An ATM switch constructs a virtual destination (VD) and a virtual source (VS) within a control loop. The VD and VS divide the control loop into two segments, namely, a segment containing the sending terminal and the VD and a segment containing the VS and the receiving terminal.
Each segment independently manages the RM cells. In a network that uses an ATM switch, however, the sending terminal and receiving terminal perform communication without recognizing the presence of the VD and VS. Specifically, the sending terminal does not recognize whether it is communicating with a receiving terminal or a VD, and the receiving terminal does not recognize whether it is communicating with a sending terminal or a VS. Therefore, the ratio of FRM cells and user cells must be equal on the VD and VS sides of the ATM switch. Similarly, the ratio of BRM cells and user cells must also be equal on the VD and VS sides of the ATM switch. The sending terminal cannot properly control the communication rate if these conditions are not met.
The quantity of flow of FRM cells sent from the sending terminal to the VD is sometimes reduced, causes of which include a malfunctioning sending terminal and malicious intent on the part of the user. In this case, essentially more user cells flow into the VD of the ATM switch than are supposed to. When there is a reduction in the flow of FRM cells, the VD of the ATM switch matches the ratio of FRM cells and user cells supplied to the VS to a preset value. Accordingly, the cell buffer of the ATM switch overflows. This overflow can have an adverse effect on other normal connections.
For instance, let us consider a case in which the band that can be used by one connection of an ABR service is 10 MHz and the ratio of user cells to RM cells is 9:1. In other words, the user cell band is 9 MHz and the RM cell band is 1 MHz. If an abnormality occurs at the sending terminal and the ratio of user cells to RM cells changes to 19:1 when an RM cell is sent, the user cell band becomes 9.5 MHz and the RM cell band 0.5 MHz. Here again, the VD sends the user cells and RM cells to the VS at a ratio of 9:1 as was specified in advance. Therefore, the VD receives the user cells at 9.5 MHz and sends them at 9 MHz. Consequently, there is excessive reception at the ATM switch, and congestion tends to occur. This congestion adversely effects the other connections of the ATM switch.
An ATM switch manages and controls cells through connection admission control, usage parameter control, or the like. Nevertheless, there are no provisions whatsoever for managing changes in the proportion of user cells and RM cells, nor has a method for managing this been developed.
It is an object of the present invention to provide a technique for preventing the overflow of an ATM exchange caused by a reduction in the flow of RM cells.
In accordance with first standpoint of present invention, the ATM exchange has an input line interface comprising means for deciding whether each ATM cell sent from a communication terminal to a switch is an FRM cell or a non-FRM cell, means for using the results of the decision to monitor the proportion of the FRM cells among the ATM cells and deciding whether the proportion is normal or abnormal, and means for discarding the non-FRM cells when the proportion is abnormal.
With the first invention, it is possible to prevent the overflow of an ATM exchange caused by a reduction in the flow of FRM cells.
In accordance with second standpoint of present invention, The ATM exchange has an output line interface comprising means for deciding whether each ATM cell sent from a communication terminal to a switch is a BRM cell or a non-BRM cell, means for using the results of the decision to monitor the proportion of the BRM cells among the ATM cells and deciding whether the proportion is normal or abnormal, and means for discarding the non-BRM cells when the proportion is abnormal.
With the second invention, it is possible to prevent the overflow of an ATM exchange caused by a reduction in the flow of BRM cells.